An internal combustion engine operates at relatively low efficiency. One reason for this is due to the circular movement of the crankshaft arm. Consequently, torque transmission from the piston is incomplete. Another indicator of this low efficiency is the relatively high temperature and pressure of exhaust gases at the output indicating a significant amount of energy from the combustion process is being lost.
U.S. Pat. No. 2,155,497 describes an example of an internal combustion engine with dual connecting rods and a rack that drives a gear.